Supernatural season 11 midseason premiere saw the return of Castiel in the most twisted way possible.

The episode opened with Lucifer offering a deal involving Sam becoming his vessel. Lucifer would in turn, take care of Amara. Besides, Dean and Castiel investigate if Amara is alive. Much to Castiel's dismay, Darkness lives, and the angel barely escapes Amara's wrath.

Dean also learns about his brother and teams up with Castiel to rescue Sam from Lucifer's Cage. A brawl ensues between Sam, Dean, Castiel and Lucifer, but the Devil vanishes in a bright light, moments before he nearly chokes Castiel to death, thanks to Rowena's spell. However, there is a twist! Unknown to the brothers, Castiel agrees to be Lucifer's vessel. So now Castiel is basically Lucifer!

Previously, executive producer Jeremy Carver had teased that Castiel will return in "new and unexpected ways" in the midseason premiere. Castiel's quest to repair his angel mojo will lead to "startling consequences" for everyone, according to the producer.

Although Dean did detect something odd about Castiel, both are unaware that the angel has a bit of the devil on his shoulder. Will Dean and Sam find out Castiel's secret in the upcoming episode? The promo released for episode 12 titled, Into The Mystic, shows the Winchester brothers doubting Castiel.

The promo opens with Dean telling Sam, "We need to find us a case." Sam asks, "What about Cas?" and Dean replies, "There is something off about him."

The promo shows Castiel walking down the forest, as an angel shows up and says, "Lucifer?" Castiel kills him with the stroke of his finger. The voice-over says, "The Devil you know, gets stranger."

The official synopsis released for the episode teased that the Winchester boys will find a case to work on the episode. Here is the full plot:

Sam (Jared Padalecki) and Dean (Jensen Ackles) investigate a case where people are suffering violent deaths after hearing a mysterious song.

Supernatural season 11 episode 11 airs on 27 January, at 9pm EST on The CW.